The use of secondary batteries (storage batteries) has become widespread over the past several years, extending across a broad range of application; e.g., cellular telephones, notebook personal computers, electric tools, electric bicycles, hybrid vehicles (HEV), and electric automotive vehicles (EV). Among these applications, nickel-hydrogen storage batteries have been used as a power source for electric tools, electric bicycles, HEVs, EVs, and other devices that require high output. An LaNi5 (AB5) hydrogen storage alloy is used as the negative electrode active material in such nickel-hydrogen storage batteries.
The amount of hydrogen stored in the hydrogen storage alloy is expressed in terms of a 1:1 ratio between the alloy and the hydrogen atoms, and it is substantially difficult to store a greater proportion of hydrogen. A dramatic increase in the quantities of secondary batteries introduced commercially with even higher capacities cannot be expected as long as LaNi5 hydrogen storage alloys are used. Conversely, with Laves phase (AB2) hydrogen storage alloys, which have a Laves phase as the main phase, it is known that one or more hydrogen atoms can be stored for each alloy, and a high-capacity secondary battery can be theoretically obtained. However, it has not yet been possible to use such hydrogen storage alloys as a negative electrode material because, e.g., a stable oxide film forms on the surface of the hydrogen storage alloy.
In contrast to the above, a recently discovered hydrogen storage alloy, whose main constituent elements are magnesium, nickel, and a rare-earth element, is characterized in having higher capacity in terms of volume and mass than the LaNi5 hydrogen storage alloy, having a higher rate of activation than the Laves phase hydrogen storage alloy, and having exceptional high-efficiency charge and discharge properties. Accordingly, using this hydrogen storage alloy makes it possible to achieve higher capacitance than the LaNi5 hydrogen storage alloy, and to yield high-efficiency charge and discharge properties that are superior to those of the Laves phase hydrogen storage alloy.
A hydrogen storage alloy of the above description, having magnesium, nickel, and a rare-earth element as main constituent elements, has been proposed in Patent Reference 1 (Japanese Laid-open Patent Application No. 11-162459). The hydrogen storage alloy proposed in Patent Reference 1 is represented by the general formula (R1-xMgx)NiyAz (wherein R represents at least one element selected from the group including a rare-earth element (including yttrium), Ca, Zr, and Ti; A represents at least one element selected from the group including Co, Mn, Fe, V, Cr, Nb, Al, Ga, Zn, Sn, Cu, Si, P, and B; and 0<x<1, 0≦z≦1.5, 2.5≦y+z≦4.5.)
In this case, x, which is the substitution amount of Mg with respect to R, is set in a range of 0<x<1, thereby remedying the problem of hydrogen not being readily released, and enabling a high discharge capacitance to be achieved. Setting the amount z of A such that 0≦z≦1.5 makes it possible to improve the rate of hydrogen storage and release as well as other characteristics of the hydrogen storage alloy; and to dramatically improve the cycle characteristics of the nickel-hydrogen storage battery. In an alkaline secondary battery comprising a hydrogen storage alloy containing an A element, the cycle characteristics can be improved, and the discharge capacity can be improved particularly when Co is used as the A element.
With the amount y+z of Ni and A contained in the hydrogen storage alloy being within a range of 2.5 or greater, the rate at which hydrogen is stored and released, and other hydrogen storage/releasing characteristics of the hydrogen storage alloy can be dramatically improved, a high discharge capacitance can be achieved, and the cycle characteristics are improved. However, when y+z is set to 4.5 or greater, the hydrogen sites in the alloy decrease in number, the hydrogen storage capacity decreases, and the discharge capacity decreases as well.
Nickel-hydrogen storage batteries as described above are being increasingly used in assisted bicycles, HEVs, EVs, and the like; and the demand for larger batteries and higher power is increasing. In view of this, a proposal has been made in Patent Reference 2 (Japanese Laid-open Patent Application No. 2005-32573) for increasing the hydrogen equilibrium pressure (P) in a hydrogen storage alloy whose main constituent elements are magnesium, nickel, and a rare-earth metal element. As is taught in Patent Reference 2, increasing the hydrogen equilibrium pressure of the hydrogen storage alloy brings about an increase in the hydrogen concentration and an improvement in the discharge characteristics (a decrease in the overvoltage). As is also taught, increasing the hydrogen equilibrium pressure of the hydrogen storage alloy brings about an increase in the open circuit voltage of the battery, and an improvement in the discharge characteristics.    [Patent Reference 1] Japanese Laid-Open Patent Application No. 11-162459    [Patent Reference 2] Japanese Laid-Open Patent Application No. 2005-32573